I'd say Latymer 6th form is exceptional. It's in North London and annually sends about 20-30 students to Oxbridge (mostly Cambridge). You might want to look into that. Btw I know most schools e.g. Henriettta Barnett as well as Latymer require 6 As, but I wouldn't aim for this as most applicants have surpassed this amount in yr 10 alone. I know someone who was predicted 5 A*s and was rejected. I don't mean to dishearten you, but unfortunately that's the reality.
Go to a really bad one and get contextual offers!!